Detailed information about the interface between actin and myosin, as well as between myosin heavy chains and light chains, is needed for a better understanding of various aspects of the actin-myosin interaction underlying the cross-bridge cycles of muscle contraction. Also, in view of the recognized role of so-called hinge regions in the in vivo motion of myosin cross-bridges, infomation on the regions corresponding to the primary structure of myosin would be desirable. Thus the application will focus on the following specific aims: 1) The primary structure of the two so-called hinge regions in myosin will be determined. 2). The conformation of different portions of myosin rod will be explored by examining the ability of sulfhydryl groups to form disulfide bonds and the susceptibility of these regions to proteolytic enzymes under various conditions. 3). The interaction between actin and myosin will be studied by identifying the interface regions using cross-linking and surface labeling techniques. 4). The interaction between mhosin heavy chains and light chains will be studied by direct cross-linking experiments and by comparing the reactivities of lysine residues of heavy and light chains in the associated and dissociated states.